Showing papers in "Journal of Physics G in 2015"

Light sterile neutrinos

Abstract: The theory and phenomenology of light sterile neutrinos at the eV mass scale is reviewed. The reactor, gallium and Liquid Scintillator Neutrino Detector anomalies are briefly described and interpreted as indications of the existence of short-baseline oscillations which require the existence of light sterile neutrinos. The global fits of short-baseline oscillation data in 3 + 1 and 3 + 2 schemes are discussed, together with the implications for β-decay and neutrinoless double-β decay. The cosmological effects of light sterile neutrinos are briefly reviewed and the implications of existing cosmological data are discussed. The review concludes with a summary of future perspectives.

The PDF4LHC report on PDFs and LHC data: Results from Run I and preparation for Run II

Abstract: The accurate determination of Parton Distribution Functions (PDFs) of the proton is an essential ingredient of the Large Hadron Collider (LHC) program. PDF uncertainties impact a wide range of processes, from Higgs boson characterization and precision Standard Model measurements to New Physics searches. A major recent development in modern PDF analyses has been to exploit the wealth of new information contained in precision measurements from the LHC Run I, as well as progress in tools and methods to include these data in PDF fits. In this report we summarize the information that PDF-sensitive measurements at the LHC have provided so far, and review the prospects for further constraining PDFs with data from the recently started Run II. As a result, this document aims to provide useful input to the LHC collaborations to prioritize their PDF-sensitive measurements at Run II, as well as a comprehensive reference for the PDF-fitting collaborations.

Models of Neutrino Mass, Mixing and CP Violation

Abstract: In this topical review we argue that neutrino mass and mixing data motivates extending the Standard Model (SM) to include a non-Abelian discrete flavour symmetry in order to accurately predict the large leptonic mixing angles and violation. We begin with an overview of the SM puzzles, followed by a description of some classic lepton mixing patterns. Lepton mixing may be regarded as a deviation from tri-bimaximal mixing, with charged lepton corrections leading to solar mixing sum rules, or tri-maximal lepton mixing leading to atmospheric mixing rules. We survey neutrino mass models, using a roadmap based on the open questions in neutrino physics. We then focus on the seesaw mechanism with right-handed neutrinos, where sequential dominance (SD) can account for large lepton mixing angles and violation, with precise predictions emerging from constrained SD (CSD). We define the flavour problem and discuss progress towards a theory of favour using GUTs and discrete family symmetry. We classify models as direct, semidirect or indirect, according to the relation between the Klein symmetry of the mass matrices and the discrete family symmetry, in all cases focussing on spontaneous violation. Finally we give two examples of realistic and highly predictive indirect models with CSD, namely an A to Z of flavour with Pati–Salam and a fairly complete A 4 × SU(5) SUSY GUT of flavour, where both models have interesting implications for leptogenesis.

Halos in medium-heavy and heavy nuclei with covariant density functional theory in continuum

Abstract: Covariant density functional theory with a few parameters has been widely used to describe the ground- and excited-state properties for nuclei all over the nuclear chart. In order to describe the exotic properties of unstable nuclei, the contribution of the continuum and its coupling with bound states should be treated properly. In this review, the development of the covariant density functional theory in continuum is introduced, including the relativistic continuum Hartree–Bogoliubov theory, the relativistic Hartree–Fock– Bogoliubov theory in continuum and the deformed relativistic Hartree– Bogoliubov theory in continuum. Then the descriptions and predictions for the neutron halo phenomena in both spherical and deformed nuclei will be reviewed. The diffuseness of the nuclear potentials, nuclear shapes and density distributions, and the impact of the pairing correlations on nuclear size are discussed.

Hyperons in neutron star matter within relativistic mean-field models

Abstract: Since the discovery of neutron stars with masses around the composition of matter in the central part of these massive stars has been intensively discussed. Within this paper we will (re)investigate the question of the appearance of hyperons. To that end we will perform an extensive parameter study within relativistic mean field models. We will show that it is possible to obtain high mass neutron stars with (i) a substantial amount of hyperons, (ii) radii of 12–13 km for the canonical mass of , and (iii) a spinodal instability at the onset of hyperons. The results depend strongly on the interaction in the hyperon–hyperon channels, on which only very little information is available from terrestrial experiments up to now.

A recipe for EFT uncertainty quantification in nuclear physics

Abstract: The application of effective field theory (EFT) methods to nuclear systems provides the opportunity to rigorously estimate the uncertainties originating in the nuclear Hamiltonian. Yet this is just one source of uncertainty in the observables predicted by calculations based on nuclear EFTs. We discuss the goals of uncertainty quantification in such calculations and outline a recipe to obtain statistically meaningful error bars for their predictions. We argue that the different sources of theory error can be accounted for within a Bayesian framework, as we illustrate using a toy model.

Top-Quark Processes at NLO in Production and Decay

Abstract: We describe the implementation of top production and decay processes in the parton- level Monte Carlo program MCFM. By treating the top quark as being on-shell, we can factorize the amplitudes for top-pair production, s-channel single-top production, and t-channel single-top produc- tion into the product of an amplitude for production and an amplitude for decay. In this way we can retain all spin correlations. Both the production and the decay amplitudes are calculated consistently at next-to-leading order in αS. The full dependence on the b-quark mass is also kept. Phenomeno- logical results are presented for various kinematic distributions at the LHC and for the top quark forward-backward asymmetry at the Tevatron.

Comparative analysis of SN1987A antineutrino fluence

Abstract: We discuss the electron antineutrino fluence derived from the events detected by Kamiokande-II, IMB and Baksan on 23 February 1987. The data are analysed adopting a new simple and accurate formula for the signal, improving on the previous modeling of the detectors response, considering the possibility of background events. We perform several alternative analyses to quantify the relevance of various descriptions, approximations and biases. In particular, we study the effect of: omitting Baksan data or neglecting the background, using simplified formulae for the signal, modifying the fluence to account for oscillations and pinching, including the measured times and angles of the events, using other descriptions of detector response, etc. We show that most of these effects are small or negligible and argue, by comparing the allowed regions for astrophysical parameters, that the results are stable. We comment on the accordance with theoretical results and on open questions.

Super-heavy nuclei

Abstract: Scientifically based searches for elements beyond uranium started after the discovery of the neutron. Neutrons captured by uranium nuclei and subsequent decay, similarly as most of the elements were produced in nature, was the successful method applied. However, as a first result, Hahn and Strassmann discovered nuclear fission indicating a limit for the existence of nuclei at an increasing number of protons. Eventually, the nuclear shell model allowed for a more accurate calculation of binding energies, half-lives and decay modes of the heaviest nuclei. Theoreticians predicted a region of increased stability at proton number Z = 126, later shifted to 114, and neutron number N = 184. These nuclei receive their stability from closed shells for the protons and neutrons. Later, increased stability was also predicted for deformed nuclei at Z = 108 and N = 162. In this review I will report on experimental work performed on research to produce and identify these super-heavy nuclei (SHN). Intensive heavy ion beams, sophisticated target technology, efficient electromagnetic ion separators, and sensitive detector arrays were the prerequisites for discovery of 12 new elements during the last 40 years. The results are described and compared with theoretical predictions and interpretations. An outlook is given on further improvement of experimental facilities which will be needed for exploration of the extension and structure of the island of SHN, in particular for searching for isotopes with longer half-lives predicted to be located in the south east of the island, for new elements, and last not least, for surprises which, naturally, emerge unexpectedly.

Symmetry broken and restored coupled-cluster theory: I. Rotational symmetry and angular momentum

Abstract: We extend coupled-cluster (CC) theory performed on top of a Slater determinant breaking rotational symmetry to allow for the exact restoration of the angular momentum at any truncation order. The main objective relates to the description of near-degenerate finite quantum systems with an open-shell character. As such, the newly developed many-body formalism offers a wealth of potential applications and further extensions dedicated to the ab initio description of, e.g., doubly open-shell atomic nuclei and molecule dissociation. The formalism, which encompasses both single-reference CC theory and projected Hartree–Fock theory as particular cases, permits the computation of usual sets of connected diagrams while consistently incorporating static correlations through the highly non-perturbative restoration of rotational symmetry. Interestingly, the yrast spectroscopy of the system, i.e. the lowest energy associated with each angular momentum, is accessed within a single calculation. A key difficulty presently overcome relates to the necessity to handle generalized energy and norm kernels for which naturally terminating CC expansions could be eventually obtained. The present work focuses on SU(2) but can be extended to any (locally) compact Lie group and to discrete groups, such as most point groups. In particular, the formalism will be soon generalized to U(1) symmetry associated with particle number conservation. This is relevant to Bogoliubov CC theory that was recently applied to singly open-shell nuclei.

Elastic and radiative heavy quark interactions in ultra-relativistic heavy-ion collisions

Abstract: Elastic and radiative heavy quark interactions with light partons are studied with the partonic transport model named the Boltzmann approach to multiparton scatterings (BAMPSs). After calculating the cross section of radiative processes for finite masses in the improved Gunion–Bertsch approximation and verifying this calculation by comparing to the exact result, we study elastic and radiative heavy quark energy loss in a static medium of quarks and gluons. Furthermore, the full 3 + 1D space–time evolution of gluons, light quarks, and heavy quarks in ultra-relativistic heavy-ion collisions at the BNL Relativistic Heavy-Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC) are calculated with BAMPS including elastic and radiative heavy flavor interactions. Treating light and heavy particles on the same footing in the same framework, we find that the experimentally measured nuclear modification factor of charged hadrons and D mesons at the LHC can be simultaneously described. In addition, we calculate the heavy flavor evolution with an improved screening procedure from hard-thermal-loop calculations and confront the results with experimental data of the nuclear modification factor and the elliptic flow of heavy flavor particles at the RHIC and the LHC.

Error analysis in nuclear density functional theory

Abstract: Nuclear density functional theory (DFT) is the only microscopic, global approach to the structure of atomic nuclei. It is used in numerous applications, from determining the limits of stability to gaining a deep understanding of the formation of elements in the Universe or the mechanisms that power stars and reactors. The predictive power of the theory depends on the amount of physics embedded in the energy density functional as well as on efficient ways to determine a small number of free parameters and solve the DFT equations. In this article, we discuss the various sources of uncertainties and errors encountered in DFT and possible methods to quantify these uncertainties in a rigorous manner.

Competition between α-decay and spontaneous fission for superheavy nuclei

Abstract: The α-decay half-lives of recently synthesized superheavy nuclei (SHN) are investigated by employing a unified fission model (UFM) and Royer's analytical formula (2000 J. Phys. G: Nucl. Part. Phys. 26 1149). The good agreement with the experimental data indicates the UFM and the analytical formula are useful tools to investigate these α-decays. A modified formula is proposed for determining the spontaneous fission half-lives based on Swiatecki's formula, including the microscopic shell correction and isospin effect.The spontaneous fission half-lives for heavy and SHN in regions from Th to Fl are calculated systematically. Experimental data are well reproduced by the modified Swiatecki formula. The competition between α-decay and spontaneous fission is analyzed in detail and the decay modes are predicted for the unknown cases.

Testing constrained sequential dominance models of neutrinos

Abstract: Constrained sequential dominance (CSD) is a natural framework for implementing the see-saw mechanism of neutrino masses which allows the mixing angles and phases to be accurately predicted in terms of relatively few input parameters. We analyze a class of CSD(n) models where, in the flavour basis, two right-handed neutrinos are dominantly responsible for the 'atmospheric' and 'solar' neutrino masses with Yukawa couplings to $({ u }_{e},{ u }_{\mu },{ u }_{\tau })$ proportional to $(0,1,1)$ and $(1,n,n-2),$ respectively, where n is a positive integer. These coupling patterns may arise in indirect family symmetry models based on A 4. With two right-handed neutrinos, using a χ 2 test, we find a good agreement with data for CSD(3) and CSD(4) where the entire Pontecorvo–Maki–Nakagawa–Sakata mixing matrix is controlled by a single phase η, which takes simple values, leading to accurate predictions for mixing angles and the magnitude of the oscillation phase $| {\delta }_{\mathrm{CP}}| .$ We carefully study the perturbing effect of a third 'decoupled' right-handed neutrino, leading to a bound on the lightest physical neutrino mass ${m}_{1}{\rm{\lesssim }}1$ meV for the viable cases, corresponding to a normal neutrino mass hierarchy. We also discuss a direct link between the oscillation phase ${\delta }_{\mathrm{CP}}$ and leptogenesis in CSD(n) due to the same see-saw phase η appearing in both the neutrino mass matrix and leptogenesis.

APFEL Web: a web-based application for the graphical visualization of parton distribution functions

Abstract: We present APFEL Web, a Web-based application designed to provide a flexible user-friendly tool for the graphical visualization of parton distribution functions. In this note we describe the technical design of the APFEL Web application, motivating the choices and the framework used for the development of this project. We document the basic usage of APFEL Web and show how it can be used to provide useful input for a variety of collider phenomenological studies. Finally we provide some examples showing the output generated by the application.

QCD resummation for hadronic final states

Abstract: We review the basic concepts of all-order calculations in Quantum Chromodynamics (QCD) and their application to collider phenomenology. We start by discussing the factorization properties of QCD amplitudes and cross-sections in the soft and collinear limits and their resulting all-order exponentiation. We then discuss several applications of this formalism to observables which are of great interest at particle colliders. In this context, we describe the all-order resummation of event-shape distributions, as well as observables that probe the internal structure of hadronic jets.

Sextic potential for $\gamma$-rigid prolate nuclei

Abstract: The equation of the Bohr–Mottelson Hamiltonian with a sextic oscillator potential is solved for -rigid prolate nuclei. The associated shape phase space is reduced to three variables which are exactly separated. The angular equation has the spherical harmonic functions as solutions, while the equation is converted to the quasi-exactly solvable case of the sextic oscillator potential with a centrifugal barrier. The energies and the corresponding wave functions are given in closed form and depend, up to a scaling factor, on a single parameter. The and states are exactly determined, having an important role in the assignment of some ambiguous states for the experimental bands. Due to the special properties of the sextic potential, the model can simulate, by varying the free parameter, a shape phase transition from a harmonic to an anharmonic prolate -soft rotor crossing through a critical point. Numerical applications are performed for 39 nuclei: Ru, Mo, Xe, Ce, Nd, Sm, Gd, Dy, 172Os, Pt, 190Hg and 222Ra. The best candidates for the critical point are found to be 104Ru and Xe, followed closely by 128Xe, 172Os, 196Pt and 148Nd.

A Bayesian approach for parameter estimation and prediction using a computationally intensive model

Abstract: Bayesian methods have been successful in quantifying uncertainty in physics-based problems in parameter estimation and prediction. In these cases, physical measurements y are modeled as the best fit of a physics-based model , where θ denotes the uncertain, best input setting. Hence the statistical model is of the form where accounts for measurement, and possibly other, error sources. When nonlinearity is present in , the resulting posterior distribution for the unknown parameters in the Bayesian formulation is typically complex and nonstandard, requiring computationally demanding computational approaches such as Markov chain Monte Carlo (MCMC) to produce multivariate draws from the posterior. Although generally applicable, MCMC requires thousands (or even millions) of evaluations of the physics model . This requirement is problematic if the model takes hours or days to evaluate. To overcome this computational bottleneck, we present an approach adapted from Bayesian model calibration. This approach combines output from an ensemble of computational model runs with physical measurements, within a statistical formulation, to carry out inference. A key component of this approach is a statistical response surface, or emulator, estimated from the ensemble of model runs. We demonstrate this approach with a case study in estimating parameters for a density functional theory model, using experimental mass/binding energy measurements from a collection of atomic nuclei. We also demonstrate how this approach produces uncertainties in predictions for recent mass measurements obtained at Argonne National Laboratory.

Realistic α preformation factors of odd-A and odd-odd nuclei within the cluster-formation model

Abstract: The recently proposed cluster-formation model (CFM) (Ahmed et al 2013 J. Phys. G: Nucl. Part. Phys. 40 065105) is extended to calculate the α preformation factors of odd–A and odd–odd nuclei. The extraction process of the formation energy of a preformed α cluster is analyzed in detail, which is crucial for determining the realistic α preformation factor in CFM. With an adaptive modification in the formation energy, we investigate the α preformation factors of odd–A and odd–odd heavy nuclei, and our results show a good agreement with both theoretical prospects and experimental extracted values. This work confirms the validity of the CFM in α preformation factor calculation and can be a useful reference for microscopic calculation in the future.

Enhancing the interaction between nuclear experiment and theory through information and statistics

Abstract: This Focus Issue draws from a range of topics within nuclear physics, from studies of individual nucleons to the heaviest of nuclei. The unifying theme, however, is to illustrate the extent to which uncertainty is a key quantity, and to showcase applications of the latest computational methodologies. It is our assertion that a paradigm shift is needed in nuclear physics to enhance the coupling between theory and experiment, and we hope that this collection of articles is a good start.

Low energy theorems of quantum gravity from effective field theory

Abstract: In this survey, we review some of the low energy quantum predictions of general relativity which are independent of details of the yet unknown high-energy completion of the gravitational interaction. Such predictions can be extracted using the techniques of effective field theory.

Analytical solutions for the Bohr Hamiltonian with the Woods–Saxon potential

Abstract: Approximate analytical solutions in closed form are obtained for the five-dimensional Bohr Hamiltonian with the Woods–Saxon potential, taking advantage of the Pekeris approximation and the exactly solvable one-dimensional extended Woods–Saxon potential with a dip near its surface. Comparison with the data for several γ-unstable and prolate deformed nuclei indicates that the potential can describe well the ground state and ${\gamma }_{1}$ bands of many prolate deformed nuclei corresponding to a large enough ‘well size’ and diffuseness, while it fails in describing the ${\beta }_{1}$ bands, due to its lack of a hard core, as well as in describing γ-unstable nuclei, because of the small ‘well size’ and diffuseness they exhibit.

Covariance analysis for energy density functionals and instabilities

Abstract: We present the covariance analysis of two successful nuclear energy density functionals (EDFs), (i) a non-relativistic Skyrme functional built from a zero-range effective interaction, and (ii) a relativistic nuclear EDF based on density dependent meson–nucleon couplings. The covariance analysis is a useful tool for understanding the limitations of a model, the correlations between observables and the statistical errors. We show, for our selected test nucleus Pb, that when the constraint on a property A included in the fit is relaxed, correlations with other observables B become larger; on the other hand, when a strong constraint is imposed on A, the correlations with other properties become very small. We also provide a brief review, partly connected with the covariance analysis, of some instabilities displayed by several EDFs currently used in nuclear physics.

Stylized features of single-nucleon momentum distributions

Abstract: Nuclear short-range correlations (SRC) typically manifest themselves in the tail parts of the single-nucleon momentum distributions. We propose an approximate practical method for computing those SRC contributions to the high-momentum parts. The framework adopted in this work is applicable throughout the nuclear mass table and corrects mean-field models for central, spin–isospin and tensor correlations by shifting the complexity induced by the SRC from the wave functions to the operators. It is argued that the expansion of these modified operators can be truncated to a low order. The proposed model can generate the SRC-related high-momentum tail of the single-nucleon momentum distribution. These are dominated by correlation operators acting on mean-field pairs with vanishing relative radial and angular-momentum quantum numbers. The proposed method explains the dominant role of proton–neutron pairs in generating the SRC and accounts for the magnitude and mass dependence of SRC as probed in inclusive electron scattering. It also provides predictions for the ratio of the amount of correlated proton–proton to proton–neutron pairs which are in line with the observations. In asymmetric nuclei, the correlations make the average kinetic energy for the minority nucleons larger than for the majority nucleons.

GT neutrino–nuclear responses for double beta decays and astro neutrinos

Abstract: Gamow–Teller nuclear matrix elements (NMEs) for pairs of ground-state-to-ground-state transitions, in particular their geometric mean NME , are studied. The observed means in the medium-heavy mass region are compared with the corresponding single-quasiparticle (qp) NMEs and the means calculated by the proton neutron qp random-phase approximation (pnQRPA). The NMEs turn out to be insensitive to the nucleon occupancy/vacancy amplitudes and to the particle–particle interaction parameter of the pnQRPA. The observed mean NMEs are found to be reduced by a coefficient relative to the effective qp NMEs and by a coefficient with respect to the pnQRPA NMEs. The reductions associated with the spin isospin correlations and nuclear medium effects, and their impact on nuclear double beta decays and astro-neutrino–nuclear interactions are discussed.

Search for Quasi Bound η Mesons

Abstract: The search for a quasi bound η meson in atomic nuclei is reviewed. This tentative state is studied theoretically as well as experimentally. The theory starts from elastic η nucleon scattering which is derived from production data within some models. From this interaction the η nucleus interaction is derived. Model calculations predict binding energies and widths of the quasi bound state. Another method is to derive the η nucleus interaction from excitation functions of η production experiments. The s wave interaction is extracted from such data via final state interaction (FSI) theorem. We give the derivation of s wave amplitudes in partial wave expansion and in helicity amplitudes and their relation to observables. Different experiments extracting the FSI are discussed as are production experiments. So far only three experiments give evidence for the existence of the quasi bound state: a pion double charge exchange experiment, an effective mass measurement, and a transfer reaction at recoil free kinematics with observation of the decay of the state.

Infrared extrapolations for atomic nuclei

Abstract: Harmonic oscillator model-space truncations introduce systematic errors to the calculation of binding energies and other observables. We identify the relevant infrared (IR) scaling variable and give values for this nucleus-dependent quantity. We consider isotopes of oxygen computed with the coupled-cluster method from chiral nucleon–nucleon interactions at next-to-next-to-leading order and show that the IR component of the error is sufficiently understood to permit controlled extrapolations. By employing oscillator spaces with relatively large frequencies, well above the energy minimum, the ultraviolet corrections can be suppressed while IR extrapolations over tens of MeVs are accurate for ground-state energies. However, robust uncertainty quantification for extrapolated quantities that fully accounts for systematic errors is not yet developed.

Statistical methods for thermonuclear reaction rates and nucleosynthesis simulations

Abstract: Rigorous statistical methods for estimating thermonuclear reaction rates and nucleosynthesis are becoming increasingly established in nuclear astrophysics. The main challenge being faced is that experimental reaction rates are highly complex quantities derived from a multitude of different measured nuclear parameters (e.g., astrophysical S-factors, resonance energies and strengths, particle and γ-ray partial widths). We discuss the application of the Monte Carlo method to two distinct, but related, questions. First, given a set of measured nuclear parameters, how can one best estimate the resulting thermonuclear reaction rates and associated uncertainties? Second, given a set of appropriate reaction rates, how can one best estimate the abundances from nucleosynthesis (i.e., reaction network) calculations? The techniques described here provide probability density functions that can be used to derive statistically meaningful reaction rates and final abundances for any desired coverage probability. Examples are given for applications to s-process neutron sources, core-collapse supernovae, classical novae, and Big Bang nucleosynthesis.

Statistical uncertainties of a chiral interaction at next-to-next-to leading order

Abstract: We have quantified the statistical uncertainties of the low-energy coupling-constants (LECs) of an optimized nucleon-nucleon interaction from chiral effective field theory at next-to-next-to-leading order. In addition, we have propagated the impact of the uncertainties of the LECs to two-nucleon scattering phase shifts, effective range parameters, and deuteron observables.

Production of heavy neutron-rich nuclei in transfer reactions within the dinuclear system model

Abstract: The dynamics of nucleon transfer processes in heavy-ion collisions is investigated within the dinuclear system model. The production cross sections of nuclei in the reactions 136Xe+208Pb and 238U+248Cm are calculated, and the calculations are in good agreement with the experimental data. The transfer cross sections for the 58Ni+208Pb reaction are calculated and compared with the experimental data. We predict the production cross sections of neutron-rich nuclei Eu, Tb, Ho, and based on the reaction 176Yb+238U. It can be seen that the production cross sections of the neutron-rich nuclei 165Eu, 169Tb, 173Ho, and 181Yb are 2.84 ?b, 6.90 ?b, 46.24 ?b, and 53.61 ?b, respectively, which could be synthesized in experiment.